Yes, DOCKAGE is a Scrabble word!

Letter values in Scrabble US: D 2, O 1, C 3, K 5, A 1, G 2, E 1.

Is it a valid word?

DOCKAGE is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

a fee charged for a vessel to use a dock
landing in a harbor next to a pier where ships are loaded and unloaded or repaired; may have gates to let water in or out
the act of securing an arriving vessel with ropes

Also: docking fee, docking facility, tying up.